Monkey kidney cells (Vero) grown in tissue culture in 35 mm Petri dishes have been observed in a calorimeter possessing long term stability of 0.5 microwatts. Cells in monolayer culture in maintenance medium were found to give a constant level of heat production that was stable for over five days. This base line characteristic of living cells was established to be exothermic relative to the identical instrumental base line exhibited by media alone and cells killed by exposure to either methanol, formaldehyde or Gamma-radiation. Infection of living Vero cells by an attenuated strain of poliovirus exhibited a broad inverse sigmoidal transition between these two regimes. This thermokinetic behavior following virus infection was found both for cell monolayers in minimal maintenance medium and for suspensions of cells in growth media. Detachment of the cell sheet and rounding up of cells, characteristic of viral infection was observed concurrently by microscopic examination. The characteristic time course of the decay in heat production was related to the amount of infecting virus suggesting that calorimetry might be utilized as an objective method for the assay of viral titre.